Solid propellant burning control systems



Aug. 16, 1966 R. w. MANN ETAL 3,266,247

SOLID PROPELLANT BURNING CONTROL SYSTEMS Filed July 1, 1963 2 Sheets-Sheet 1 Robert W. Mann William S. Griffin Charles H. Mursfon INVENTORS Attorney 1965 R. w. MANN ETAL 3,266,247

SOLID PHOPELLANT BURNING CONTROL SYSTEMS Filed July 1, 1953 2 Sheets-Sheet 2 DISCHARGE 38 5 GAS OUTPUT GAS OUTPUT 6o 67 6| 5 64 EXHAUST 5a 57 54 55 A Ar l 63 H ez Robert W .Mc|n1n V V William S.Griffin 4/ d1 Charles H. Mqrsfon 7 3 70 I P I N PUT INVENTORS BY F lg. 5

Aftorn United States Patent 3,266,247 SOLID PRGlPELLANT BURNING CUNTRGL EBYSTEMS Robert W. Mann, Lexington, Mesa, William S. Grillin, Fairview Park, Chic, and Charles H. Marston, Eerwyn, Pa, assiguors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed July 1, 1963, Ser. No. 292,170 6 Claims. (Cl. 60--39.02)

The present invention relates generally to combustion systems and processes and, more particularly, to apparatus for and methods of controlling the burning rate of solid fuel elements.

It is comomn practice in guided missile systems to utilize a gas generator for powering the auxiliary electrical generating equipment and for activating the various attitude control mechanisms. Because of their storage stability, compactness, high reliability and readiness for instant use, solid propellants are frequently employed as the fuel source for these generators.

The demands on these gas generators may vary considerably during their period of operation. One type of regulation currently used to control the output of the gas generator simply involves burning the propellant at a mass burning rate corresponding to the maximum expected demand and dumping the excess gas produced during periods of lower consumption.

Another approach resorted to for achieving this control involves varying the mass burning rate of the solid pro pellant by changing its surface burning area. For example, the propellant grain, which may have the configuration of a solid, circular cylinder, is advanced into the combustion chamber so that it burns in the cigarette mode with the burning confined to the end face thereof. When the demand increases, the grain is moved into the chamber at a rate in excess of its linear burning rate. This exposes the wall portions of the propellant grain to the combustion process, and the burning area expands to a conical surface with the output increasing proportionally.

The first mode of operation mentioned above, it will be recognized, is inefiicient and wasteful of fuel, while the second technique does not recommend itself because of the limited degree of control that can be obtained and the uncertainty of this control.

The burning rate of solid propellants, as is well known, is sensitive to the temperature of the propellant prior to its ignition, varying typically by as much as a factor of two over the range from 65 F. to +150 F. Because of this temperature sensitivity, the designer of a power supply using these propellants is faced with the choice of either controlling this soak temperature or providing a propellant grain sufficiently large to meet the highest temperature that may be encountered, along with some provision for expending the surplus gas power. While the weight of propellant wasted in the latter case is of little consequence where low power levels are encountered, this waste does, however, become a significant part of the total weight of the power supply until when, for example, the gas generator is being employed as the main propulsion means for submarine torpedoes. Thus, the controlling of the burning rate of solid propellants is highly desirable in situations where variable temperatures are encountered and where demand varies with time.

The primary object of the present invention is to provide a new and improved method for controlling the burning rate of a solid propellant.

Another object of the present invention is to provide an arrangement for modulating the output of a gas generator having as its fuel element a solid propellant grain.

A further object of the present invention is to provide a method for controlling the burning rate of a solid propellant grain by regulating the heat transfer from the flame zone back to the gas-solid interface of the solid propellant.

A yet still further object of the present invention is to provide a method for continuously varying the burning rate of an end burning, solid propellant.

A yet still further object of the present invention is to vary the burning rate of an end burning, solid propellant by means of a ram heated by the products of combustion and positioned in close proximity to the gas-solid interface.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 schematically illustrates the operating principle of the present invention;

FIG. 2 shows a ram for use in the control system;

FIG. 3 shows the face portion of the ram of FIG. 2;

FIG. 4 illustrates one embodiment wherein the motive power for moving the solid propellant against the ram is derived from the combustion process; and

FIG. 5 illustrates another embodiment wherein the solid propellant grain is moved by an independent source of pressurized gas.

Solid propellants fall into two broad classifications with fundamentally difierent ways of burning. Double-base propellants consist of approximately nitrocellulose, 40% nitroglycerin and 10% stabilizer, plus other additives. The double-base propellant is an essentially uniform, colloidal plastic which decomposes in several stages to N C0, C0 H and H 0 plus carbon particles. Composite propellants, the other class, consist of a physical mixture of an oxidizer, such as ammonium nitrate or potassium chlorate, in a matrix of rubber, polyester, vinyl, polymer or other fuel. Complete combustion with composite propellants requires gaseous diffusion of oxygen-rich and oxygen-deficient components. The present invention has application to both types of solid propellants.

The burning of a double-base solid propellant may be divided into several distinct zones. They include a subsurface .and surface zone, a so-called fizz zone, a dark zone Where the intermediate products of combustion exist and where the temperature is approximately constant between 600 F. to 1000 F., and a flame reaction zone where the temperature increases from a low of approximately 600 F. to a high of 2800 F. In the subsurface, surface and fizz zones, the temperature increases from the soak temperature of the solid propellant to approximately the 600 F. level mentioned above.

The burning surface and its temperature are, of course, difficult to define because of the violent physical and chemical changes there taking place. However, it has been estimated that under moderate pressure this temperature is in the neighborhood of 300 C. or about 600 F. At pressures below 2.00 lbs/sq. in., the flame is nonluminous. With increasing pressures, the flame first become-s luminous at some distance from the burning surface and gradually approaches the surface until at 1,000 lbs./ sq. in. the space between the flame and surface is difficult to detect.

In analyzing the solid propellant burning process, Huggett, in his book, Solid Propellants and Rockets, Princeton University Press, 1960, suggests that at high pressures most of the energy necessary to bring about the surface decomposition (which leads to a regression of the propellant surface and thus establishes the burning rate) comes from the high temperature flame region. The

outside circumference for maximum effectiveness.

seesaw rate of energy transfer tothe surface will be determined by the thermal conductivity of the flame and the proximity of the high temperature region to the burning surface. Since the rates of the flame reactions are strongly pressure dependent, this will lead to increased energy transfer and thus to a smoothly increasing burning rate as the pressure increases.

The present invention is based upon the concept that the burning rate of a solid propellent can be modulated by influencing the heat flow from the flame zone back to the propellant gassolid interface. It has been experimentally determined that this burning rate can be continuously varied at well over a range up to almost ten times the equilibrium vented vessel burning rate by introducing a ram into the combustion chamber and pushing this ram, after it has been properly heated, into close proximity to the gas-solid interface. The increase in burning rate so realized, it has been found, is approximately proportional to the force applied to the ram and to the total perimeter of the ramming surface. To be effective as a modulator, this ram must be heated by the products of combustion to a temperature level substantially higher than the above interface temperature. By

utilizing a ram with thin wall sections, for example, the

time delay from propellant ignition to the attainment of this temperature may be reduced to less than a few seconds.

In an alternative mode of operation, the ram is maintained stationary within the combustion chamber and the propellant grain is forced up against it. With such an arrangement, the problem of sealing the ramrod where it enters the combustion chamber is avoided. Also, the over-all length of the apparatus is substantially reduced.

Since the increase in the burning rate is proportional to the applied force per unit area covered by the ram and the total perimeter of the ramming surface, the ram preferably should have a high ratio of surface perimeter to The ram also should be made of a material which insures a high heat transfer to it from the flame zone. Additionally, the ram should have good conductivity and a low thermal mass. Molybdenum, for example, meets the material specifications.

The term equilibrium burning, as used hereinafter, refers to the quasi steady-state burning reached by the end burning, propellant grain used in the present invention when positioned in a combustion chamber which exhausts the hot gases through a constant area nozzle.

The fundamental operating principle of the present invention can be understood by referring now to the apparatus shown in FIG.1. In this solid propellant, burning rate control system, a solid propellant grain I, having the geometry of a right circular cylinder, is accommodated within the lower portion of a combustion chamber 2. This grain, it will be appreciated, is ignited 'by a suitable squib, not shown. Also positioned within combustion chamber 2 so as to be heated by the combustion process is a ram 3 in the form of a right circular cylinder. This ram, whose design will be described hereinafter in greater detail, is mounted on one end of a rod 4 which, in turn, is attached to an air-actuated piston 5 housed within a cylinder 6. Between ram 3 and piston 5, rod 4 passes through a removable insert 7 mounted in the upper wall of the combustion chamber. This insert, which has an aperture slightly larger than the diameter of the rod, performs with this rod as an annular nozzle for venting the combustion gases into an upper compartment 8 from which they pass via ports 9 to the utilization device. Suitable blast deflectors 10 are also located within upper compartment 8 to direct the exhaust gases through these ports.

In order to increase, for example, the burning rate of grain 1, once it has been ignited and has reached its equilibrium burning rate, the present invention merely forces a solid body, here exemplified by ram 3, into close proximity to its end burning surface. The movement of this ram is achieved via the air-operated piston 5.

The ram influences the burning by setting up thermal gradients within the system and by changing the heat transfer conditions normally existing therein. It is fairly clear that the ram effects the surface reaction and that the heat transfer from the flame zone plays an important part in the burning rate acceleration process. The heat from the flame zone is undoubtedly transferred to the ram and, thence, to the burning surface. It has been observed that significant burning rate increases occur only when the ram has been heated to approximately 1,000 F. or more. A ceramic ram had little effect on the burning rate. Refractory metals, in particular molybdenum, appear to be the most effective materials for the ram. In general, the ram should be capable of high heat conduction, possess a high melting temperature and a freedom from brittleness.

The ram design shown in FIG. 2 meets the other requirements mentioned above. As seen in this figure, this ram consists of a molybdenum block 24) having a plurality of seamless molybdenum tubes, such as 21 and 22, projecting from one face thereof. These tubes, which serve as the ramming surface, are press-fitted in place and are uniformly distributed in a triangular pattern over an area corresponding to the propellant grain cross section. Communicating with these tubes and forming passageways for the hot gas flow are a similar number of apertures 23, 24, etc.

As perhaps best shown in FIG. 3, which is a head-on view of a portion of the ram, a second pattern of apertures, such as 25 and 2d, symmetrically positioned with respect to each of the triangles, is cut through block 20 for increasing the gas flow therethrough.

When a ram of the type shown in FIG. 2 is forced up against the burning surface of an end-burning propellant, the open end portions of the various tubes pierce the fizz zone referred to previously and penetrate into the subsurface zone where the propellant has a viscous quality. The heat transfer from the flame zone now takes place not only by convection but also by conduction through the individual tube walls. This heat transfer, it would be observed, is not only via the end rims of these tubes but also via the adjacent inner and outer tubular wall surfaces'which contact the propellant material.

As mentioned hereinbefore, from an engineering standpoint it is much more feasible to move the solid propellant grain rather than the ram. FIG. 4 schematically depicts a feedback controlled hot gas generator employing this mode of operation. Here, the solid propellant grain 30, inhibited so as to burn in the cigarette mode, is coaxialy positioned within a cylindrical housing 3-1 whose inner diameter is made greater than that of the grain so as to provide an annular stroage space 32 for an incompressible fluid 33. Grain 30 is supported adjacent its burning end by an inner collar 34 'which also serves as a common wall between annular storage space 32 and combustion chamber 35. The other end of grain 30 fits into one end of a piston 36 which is provided with an O-ring seal 37. An O-ring seal 38 is also included in the inner surface of collar 34.

The main control mechanism for this gas generator, valve 39, is located in a discharge line 40 which communicates with the annular storage space 32. This valve, Whose condition is set by the demand, regulates the discharge of liquid 33 and, in doing so, determines the rate at which grain 30 can be advanced into the cornbustion chamber and the force which it can exert on the ram.

Positioned within combustion chamber 35 and fixed at a location adjacent the end wall 4-2 of cylinder 31 is a ram 43 of the type shown in FIG. 2. This ram does not impede the flow of the gas within this chamber or out via nozzle 44 to the utilization device because of the various axial passageways cut through its mounting block.

To control the advancement of piston 36 and the force with which the burning surface of grain 30 is pressed against the various tubular elements of the ram, a feedback path 45 is connected between the combustion chamber and the end portion 46 of cylinder 31 behind piston 36. The stagnation of the combustion gases in feedback tube 45 and in the actuation volume 46 plus the heat sink action of this tube, piston 36 and cylinder 31 combine to reduce the temperature of these gases to a tolerable level.

' Because the head area of piston 36 is larger than the grain cross-sectional area, unequal forces act on opposite ends of grain 30 during its burning cycle. Thus, this grain will move into the combustion chamber whenever valve 39 permits it to do so. During normal or equilibrium burning conditions, it will be appreciated, valve 27 releases only enough liquid to advance the grain at its linear burning rate. When the demand on the system increases, however, this discard rate is increased and the grain is brought up against the ram with sufficient pressure to increase the burning rate to the level required. Likewise, when the demand decreases, the liquid discard rate isreduced or stopped. The fuel combustion interface burns away from the ram and the normal burning rate resumes.

It has been found that with a Ceres 4C type of propellant grain and with the ram design shown in FIG. 2 a piston diameter 2% greater than the grain diameter provides a force sufficient to increase the burning rate by a factor of five.

FIG. 5 depicts an alternative embodiment of the present invention where the actuating piston cooperating with the propellant grain is controlled, not by the combustion gases, but by an independent source of compressed gas. In this system combustion chamber 50 again accommodates a cylindrical, inhibited, end-burning, solid propellant grain 51, backed up by a piston 52.

The advancement of grain 51 against ram 53 is regulated in this apparatus by a standard four-way control air valve 54 which includes a housing 55 and a piston 56 disposed therein. This valve has an input line 57 which is connected to a source of compressed gas 60, an output line which is coupled via line 61 to the backside of piston 52, and an exhaust line 59. The position of piston 56, the control element of the valve, is determined in opposite ways by the demand requirements and by the combustion chamber pressure. More particularly, the remote end of this piston has attached thereto an extension rod 62 which terminates at the midpoint of a cross bar 63. One end of this cross bar is mechanically coupled by a link 64 to the top of an expandable pressure chamber 66. This chamber may, for example, take the form of a bellows having a suit-able access opening 66 to which the stagnation gases are coupled via line 67 extending from the lower wall of combustion chamber 6 8. The other end of cross bar 63 is adapted to be set or positioned in accordance with the demand requirement. Thus, when the demand increases, piston 56 is moved to the right, as seen in this figure, whereas when the combustion chamber pressure increases, this piston is moved in the opposite direction.

To activate the gas generator, piston 56, it will be appreciated, is moved to the right from the position shown and the seal between the rim portion 71 of its enlarged end 72 and the inner wall of the reduced diameter portion 70 of housing 55 is broken. Gas now flows from source 60 through valve 54 and via line 61 to the backside of piston 52. Valve 54, it will be recognized, is open only to the point where the pressure acting on the backside of piston 52 is balanced by the combustion pressure acting on the burning end of grain 51. Consequently, grain 51 remains stationary and the system burns in the unrammed mode.

To shift the system to the rammed burning mode, it is only necessary to increase the opening of valve 54 by further displacing piston 56 to the right from the position shown. This movement increases the pressure on the backside of piston 52, and grain 51 advances towards ram 53 until its burning surface is pierced by the various tubular elements of the ram.

To reduce the burning rate to a lower rammed level, valve 54 need only be throttled back from the above condition. Likewise, the system can be restored to its unrammed burning rate by simply returning this valve to its original operating position.

When grain 51 is pressed against ram 56 and the system transformed into the rammed burning mode, the pressure within combustion chamber 68 increases. To avoid the excess chamber pressures which may accompany relatively high burning rates, the apparatus also contains a pressure responsive negative feedback control feature. More particulraly, as the pressure within combustion chamber 68 increases, this increase is coupled via line 67 to chamber 65, causing it to expand and move piston 56 via linkage 64, cross arm 63 and rod 62 in a direction opposite to that which caused this increase. Valve 5'4 is thus throttled back and the force applied to grain 51 somewhat reduced from its previous level. The stiffness of bellows 65 and the geometry of the linkages coupling it to piston 56 determine the degree of feedback control acting in the system.

In connection with this modification, it would be mentioned that no combustion chamber gases flow through the control valve 54. Also, the actuating gas can either be stored in a container or generated by a special squib. Additionally, the actuating gas available should be ade quate to pressurize a volume, equal to the fuel volume, to the pressure level equal to the highest actuation pressure anticipated, with an allowance for the additional actuation gas exhausted in previous on, off cycles of the apparatus.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A hot gas generator comprising, in combination,

a combustion chamber having an outlet in one wall thereof for delivering combustion gases produced therein to a utilization device;

a solid propellant grain accommodated within said combustion chamber;

a ram fixedly positioned within said chamber, said ram having a plurality of tubular elements projecting from one face thereof;

and means for forcing said solid propellant after its ignition against said ram whereby the open ends of said tubular elements pierce the gas-solid interface of said solid propellant, said tubular element thereafter acting as heat transfer means for conducting heat from the flame zone back to the gas-solid interface of said solid propellant grain.

2. A hot gas generator comprising, in combination,

a hollow cylindrical housing closed off at each end;

a gas outlet formed in one end wall of said housing;

a cylindrical solid propellant grain concentrically positioned within said housing, said grain having a diameter less than the inside diameter of said housing whereby an annular space exists between the outer wall surface of said grain and the inner wall surface of said housing;

a piston accommodated within said housing, said piston supporting one end of said grain with a rim portion thereof closing off one end of said annular space;

sealing means for supporting the other end of said grain;

said sealing means also closing off the other end of said annular space;

an incompressible fluid stored within said annular space;

a ram positioned in said housing between said sealing means and said one end wall;

means for feeding some of the gases produced during the combustion of said grain from said combustion chamber to the backside of said piston thereby to urge said grain into said combustion chamber;

and means for permitting a controllable amount of said incompressible fluid to be discharged from said annular space thereby to regulate the movement of said grain into said combustion chamber and against said ram.

3. In a method for modulating the burning rate of a solid propellant grain burning in the cigarette mode, the

steps of placing a ram made of a material having a high heat conductivity in the flame zone of said burning solid propellant grain thereby to heat said ram to the temperature of said flame zone and thereafter moving the head of said heated ram against the gassolid interface while maintaining the body portion of said ram in said flame zone, thereby to produce a heat transfer by conduction from said flame zone to said gas-solid interface and cause the burning rate of said solid propellant grain to be raised. 4. In a gas generator, the combination of a combustion chamber having an outlet for delivering combustion gases produced therein to a utilization device; a solid propellant grain accommodated within said combustion chamber; a ram fixed in position within said combustion chamber at a location adjacent said outlet, said ram including a multiplicity of longitudinal elements made of a material having a high heat conductivity, said elements having a linear dimension at least eual to the distance separating the flame zone and the gas solid interface of said solzi-d propellant grain when said solid propellant grain is burning; and means for moving the gas-solid interface of said solid propellant grain when said solid propellant grain is burning against said ram to cause said ram to be heated to the temperature of said flame zone and thereafter to cause a heat transfer from said flame zone back to said gas-solid interface by means of said longitudinal elements.

5. In a hot gas generator of the type wherein the fuel element is a solid propellant grain, the combination of a combustion chamber having a nozzle in one Wall portion thereof for coupling the combustion gases produced therein to a utilization device;

a solid propellant grain accommodated within said combustion chamber;

a ram made of a material having a high heat conductivity positioned within said combustion chamber,

said ram including a base member and a multiplicity of tubular elements projecting therefrom toward one end of said propellant grain, said base member being perforated with a multiplicity of axial passageways for allowing combustion gases to flow therethrough;

and means operative by a portion of the combustion gases produced in said combustion chamber after ignition of said propellant grain for moving said grain until said ram is located between the flame zone and the gas-solid interface of said propellant grain, said ram thereafter increasing the heat transfer from said flame zone to said gas-solid interface and causing said propellant grain to burn at a higher burning rate.

6. In an arrangement as defined in claim 5 wherein said means operative by a portion of the combustion gases produced in said chamber after ignition for moving said propellant grain includes a feedback path interconnecting opposite ends of said combustion chamber.

References Cited by the Examiner UNITED STATES PATENTS 2,703,960 3/1955 Prentiss 35.6 3,022,735 2/1962 Eberle 60-356 3,073,113 1/1963 Faught 6035.6 3,105,352 10/1963 Corbett 60--35.6 3,133,410 5/1964 Gessner 60-35.6 3,182,451 5/1965 Messerly 60-35.6

MARK M. NEWMAN, Primary Examiner.

D. HART, Assistant Examiner. 

1. A HOT GAS GENERATOR COMPRISING, IN COMBINATION, A COMBUSTION CHAMBER HAVING AN OUTLET IN ONE WALL THEREOF FOR DELIVERING COMBUSTION GASES PRODUCED THEREIN TO A UTILIZATION DEVICE; A SOLID PROPELLANT GRAIN ACCOMMODATED WITHIN SAID COMBUSTION CHAMBER; A RAM FIXEDLY POSITIONED WITHIN SAID CHAMBER, SAID RAM HAVING A PLURALITY OF TUBULAR ELEMENTS PROJECTING FROM ONE FACE THEREOF; AND MEANS FOR FORCING SAID SOLID PROPELLANT AFTER ITS IGNITION AGAINST SAID RAM WHEREBY THE OPEN ENDS OF SAID TUBULAR ELEMENTS PIERCE THE GAS-SOLID INTERFACE OF SAID SOLID PROPELLANT, SAID TUBULAR ELEMENT THEREAFTER ACTING AS HEAT TRANSFER MEANS FOR CONDUCTING HEAT FROM THE FLAME ZONE BACK TO THE GAS-SOLID INTERFACE OF SAID SOLID PROPELLANT GRAIN. 